In a radio base station, there is a need for a high power amplifier in the transmitter section to provide each radio channel with enough power to reach the outer limits of a cell covered by a base station.
When using the same amplifier for simultaneous amplification of several information signals modulated on different carrier waves or when using linear modulation, such as QAM (Quadrature Amplitude Modulation), high amplifier linearity is required. This is because it is essential in this case to maintain all phase and amplitude positions of the signal components involved in the amplification. Otherwise inter-modulation will occur between the signal components and the spectrum of the amplified signal will broaden. Intermodulation distortion will interfere with signals on other channels. Examples of systems, wherein high linearity is required, are those in which several combined narrow-band signals or wide-band signals such as CDMA need to be amplified.
RF power amplifiers are inherently non-linear devices since they generate unwanted intermodulation products, which manifest themselves as spurious signals in the amplified output signal, separate and distinct from the input signal. The distortion introduced by an RF amplifier causes the phase and amplitude of its amplified output signal to depart from the phase and amplitude, respectively, of the input signal.
There exist different methods for linearising amplifiers. An overview and reference list of linearisation methods, including Back-off of class A, Feedforward, Vector summation, Feedback and Predistortion Linearisation, is given in “Linearisation of RF Power Amplifiers” by Mark. A. Briffa, department of Electrical and Electronic Engineering, Victoria University of Technology, December 1996, Australia.
Predistortion is a method that modifies the original input signal to the power amplifier such that it is complementary to the distortion characteristics of the power amplifier. The cascaded response of complementary predistortion and amplifier distortion should therefore result in a linear response. Typically the complementary predistortion function is based on the approximation of the amplifier being modelled by a power or a voltera series (otherwise known as the AM/AM & AM/PM). However, the complementary predistortion function could also include higher order effects, such as thermal properties of the transistor and/or frequency dependent properties due to the bias and matching circuits. Predistortion can be applied at radioor Intermediate frequencies, known as analog predistortion, or applied at baseband, known as digital predistortion. Additionally, to achieve excellent linearity, adaption of the predistortion means is needed to circumvent changes due temperature, component ageing etc.
The predistortion mechanism may be made adaptive by extracting the error distortion signal component at the output of the RF amplifier and adjusting the predistortion means in accordance with the extracted error behaviour of the RF amplifier during real-time operation, so as to effectively continuously minimize distortion at the amplifier's output. A good overview of digital predistortion adaptive means is given in J. K. Cavers, “Amplifier linearisation by adaptive predistortion”, U.S. Pat. No. 5,049,832. International patent applications WO 99/45638 and WO 99/45640 provide examples of an analog predistortion adaptive method.
To achieve good results with analog predistortion, careful matching of the complementary predistortion function and amplifier distortion function is required. However, analog elements with excellent matching characteristics are very difficult and expensive to produce. Analog predistortion typically has a limited dynamic (operation)range and is therefore only able to produce limited performance improvements. Analog predistortion, however, can operate over large bandwidths.
On the other hand, adaptive digital predistortion is able to produce, in theory, the excellent matching properties required to achieve significant improvements in linearity. However, because the predistortion occurs at baseband, it needs to be converted to radio frequency before it can be amplified. The frequency conversion, however, takes place in the analog domain, which by its nature destroys the perfect matching capabilities of the digital domain. This results in that also digital predistortion becomes limited in respect of performance and bandwidth compared to analog predistortion.
The frequency dependent properties of the analog frequency conversion process, amplitude ripple and phase ripple, are the basic mechanisms for destroying the digital domain's matching qualities. Both of these quantities are directly linked to the amount of performance improvement that can be obtained. As the amplitude and phase ripple quantities increase, the obtainable linearity performance improvement becomes reduced, regardless of how well matched the complementary function is in the digital domain.
Analog and digital predistortion linearisation are typically implemented as standalone techniques, because of practical implementation problems arising from the fact that the two solutions require different architectures for implementation. This is best illustrated in the following two examples. In a normal macro base station, the Power Amplifier (PA) is a module that fits in one part of the rack, sometimes included with the frequency conversion circuits. The digital base band processing circuits physically reside in another part of the rack. These two components are physically separated and the signals are communicated via a coaxial cable. In a mast mounted antenna application, the PA is at the top of the mast and the base band processing part is at the base of the mast. In both examples, the digital baseband processing parts and the PA module are physically separated, which introduces practical implementation problems.
However, there exist some examples of a combined linearisation solution. For example the combination of digital predistortion with feedforward is presented in WO 98/12800. The feedforward architecture requires additional signal paths and two couplers, one to acquire the signal out of the power amplifier (PA) and the other one to subtract the remaining PA distortion. The last coupler inserts a loss. This results in the overall efficiency being degraded, also when combining the two methods. The solution is also quite complicated, requiring a large amount of components, making it difficult to implement.
Another example of combining RF predistortion with Feedforward is described in the technology white paper, Multi-Carrier Power Amplifiers join the Digital Revolution' Yuval Shalom, Fall 1999 issue of Site management and Technology Magazine, Intertec Publishing. It states that the DC-RF efficiency of feedforward amplifiers is limited to around 6–8% because of three factors.
1) Large backoffs due to the large peak to average ratios of multicarrier (wideband) signals. The DC power supplied to the power amplifier is given by PDCamp, and the RF output power from the amplifier is given by PRF.
2) Additional hardware such as delay lines and couplers after the power amplifier introduce insertion (power) loss. The power loss can be defined as a fraction of the total power surviving after the loss, typically 0.8 i.e 20% (1 dB) of power is lost.
3) DC power consumed (PDCother) by other parts of the feedforward system i.e. error amplifiers, control circuitry.
The DC-RF efficiency formula can be approximated for the combined feedforward and RF predistortion system asDC-RF efficiency=0.8*PRF/(PDCother+Pdcamp)
In the case of a predistortion system only, there is no need for the additional hardware that introduces losses in a feedforward system. As regards the other parts of the feedforward system, this reduces complexity and results in the DC-RF efficiency increasing as shown below by the DC-RF efficiency formula for predistortion:DC-RF efficiency=PRF/PDcamp>0.8*PFR/(PDCother+PDcamp)
Wide band applications partly have to be designed with architectures and solutions different from those of the narrow band solutions because of different problems.
Predistortion is a quasi-closed loop power amplification system, as opposed to a closed loop system. Closed loop systems are based on feedback, i.e. Cartesian feedback, envelope feedback, polar feedback.
The advantage of quasi-closed loop systems is that both narrowband and wideband systems of an order of magnitude of 10's of MHz can be used. As opposed to this, closed systems are limited to narrow band signals of an order of magnitude of 100's of kHz for practical stability reasons, although they can usually achieve higher levels of linearisation than quasi-closed loop systems.
In addition to quasi-closed and closed loop systems, feedforward amplification can be used that can result in both high levels of linearity and transmittance of wideband signals. As compared to predistortion methods this requires a rather complicated gain and phase tracking mechanism to keep the loops in balance and results in low power efficiency. The feedforward architecture is currently the most widely used technique for linearising applications in wideband radio. For wideband radio applications, predistortion, both analog and digital methods, in general is characterized by good power efficiency, as compared to the use of the feedforward methods, although they have lower linearity performance than feedforward.
Digital predistortion has not been technologically possible to use for wide band signals earlier with a sufficient performance because of a relatively slow digital signal processing and limited precision. The development of faster digital signal processing by the introduction of e.g. digital circuit fabrication methods, and advancements in DAC (Digital to Analog Converter) technology, has, however, remedied this problem.
However, use of digital predistortion techniques in wide band systems accentuates problems appearing in connection with upconversion of the base band signal to a radio signal, since the frequency dependent properties of the analog frequency conversion process limit performance improvement.
The analog frequency upconversion process is usually assumed to have the same transfer function regardless of frequency, viz.:Y=fn(X)wherein Y is the output signal, and is a function of the input signal X. Typically the frequency conversion circuitry is designed such that Y is linearly related to X, and this assumption is relevant to narrow band signals. In wideband signal applications, the frequency upconversion process becomes more dependent on frequency:Y=fn(X,frequency)
This frequency dependence destroys perfect digital predistortion matching properties, and therefore limits the amount of linearisation. The dependence on frequency can be reduced, but it becomes expensive, if not impractical, to design over a wide bandwidth with very low amplitude and phase ripple. That is, low enough amplitude and phase ripple allowing digital predistortion to achieve the desired level of linearity. A digital frequency equaliser could be used to negate the frequency upconverter's frequency dependence. However, there will always exist some residual frequency dependence that limits the amount of linearisation, and it will also increase the complexity of the solution.
It is an object of the invention to provide an improved linearisation method by using digital predistortion in both wide band and narrow band applications, that will overcome the frequency conversion problems in connection with digital predistortion.